The present invention relates generally to computed tomography (CT) imaging systems. More particularly, it relates to a method and apparatus for cooling the electronics associated with CT scanners.
In at least some CT imaging system configurations, a stationary floor-mounted frame includes an x-ray source and a radiation detector array. The x-ray source projects a fan-shaped beam that is collimated to lie within an X-Y plane of a Cartesian coordinate system and is generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon the array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam of the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile. The x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged. The X-ray source typically includes an x-ray tube that emits an x-ray beam. The X-ray detectors typically include a collimator for collimating x-ray beams received at the detector. A scintillator is located adjacent the collimator and photodiodes are positioned adjacent the scintillator.
The power output by both the electronics associated with sensitive X-ray detectors and other components associated with CT imaging systems, including the x-ray tube itself, is increasing with each new generation of CT system. This increase in power output occurs while the packaging space remains constant or is decreasing. In particular, the electronics used in converting x-ray input to a useful electronic signal are rapidly becoming the dominant heat source within a CT system, rivaling the x-ray tube at steady state. Cooling of those electronics is made difficult for a number of reasons.
In order to accommodate higher signal densities and to reduce electronic noise, the electronics have been moved closer to the x-ray detector. Therefore, the high power electronics are conductively coupled to the highly temperature sensitive photodiode in the x-ray detector. Also, very little space is available between the analog to digital converters, which makes direct convective cooling difficult, if not impractical.
The photodiodes customarily employed in existing CT detectors only operate within a narrow range of temperatures. Therefore, any thermal control solution must not only prevent the heat from the electronics from reaching the photodiode but prevent the possibility of over cooling the photodiodes.
There are two factors that make direct convective cooling difficult in a CT detector gantry. The first is the fact that the air temperature within the gantry typically varies proportionally with the room ambient temperature. In general, the temperature of a room in which a CT imaging machine is used is specified and has a range of 11° C. Variation in cooling air temperature can lead to situations where the hot side heat is flowing to the photodiodes and may raise the temperature of the photodiodes above their set point temperature, which is, in general 36° C.±1° C. At the other extreme, the lowest cooling air temperature may cool the photodiode beyond its desired, or optimum operating temperature. The second factor hindering direct convective cooling is the fact that the electronics on a CT system rotate. This rotation tends to cause a decrease in airflow rate through any cooling fans that may be attached to the rotating side due to changes in the fan inlet flow boundary conditions. That is, because the inlet velocity vector tends towards being perpendicular to the flow direction, flow near the gantry covers produces lower pressure at the fan inlet. The rotation of the gantry also tends to cause mixing of the air inside the gantry thereby leading to a nearly instantaneous step change in the cooling air temperature. Another factor that needs to be considered in a direct convection cooling system is that, by virtue of allowing air to flow over the electronics, an opening has been created for the entrance of electronic fields. Thus, direct convection cooling has a negative impact on electromagnetic interference (EMI) shielding.